Mast cells of the beige mouse contain large intracellular secretory granules (approximately 4 microns in diameter) whose membranes fuse with the plasma membrane in a process called exocytosis. During membrane fusion an exocytotic pore forms which connects the granule interior with the extracellular medium. Through the exocytotic pore the granule contents are released extracellularly and are free to diffuse to target cells. We have correlated electrophysiological and light microscopic data to investigate the structure of this exocytotic pore during secretion in mast cells from Beige mice. The time course for the widening of the pore is highly variable: it can widen quickly or slowly and can fluctuate between dilated and contracted states of variable conductance (flickering). Initial pore sizes are broadly distributed indicating that this pore is different from traditional membrane channels which have a relatively fixed conductance. The frequency histogram for occurrence of pores of given conductance is broad with a primary peak between 1 and 4 nS, indicating that pore size does not increase in quantal steps. A secondary maximum occurs at about 30 nS. We have searched fast-frozen, freeze-fracture replicas of rat mast cells and identified pores with small lumens. The 30 nS pores may represent the smallest pores seen in transmission electron microscopy. A model describing fusion on the molecular level which can account for the variable pore sizes, flickering, and known volumes of activation is described. In a second project isolated matrices of the giant secretory vesicles of Beige mouse mast cells were examined to determine the effects of the ionic composition of the bathing solution on their size. In general multivalent cations condense the matrix relative to univalents.